(1) Field of the Invention
The present invention relates to an electrical filter and more particularly to a device for selectively eliminating the electromagnetic interference (EMI) effects produced by a high frequency (HF) transmitter on a proximate, very low frequency (VLF) receiver.
(2) Description of the Prior Art
High frequency EMI often interferes with the operation of VLF receivers when HF transmit and VLF receive antennas are located in close proximity thereto, such as occurs in submarine sails. Previously, a single-ended, seven pole, elliptic function filter was installed internally in the balanced input, single-ended output antenna coupler which interfaces the VLF receiver to its loop antenna. This filter was intended to act as a low pass EMI filter to prevent HF band signals from interfering with VLF reception.
Such a filter design however was found to not work. Instead, radio operators must, when the submarine platform is operating at periscope depth, lower the mast which carries the VLF loop antenna until it is submerged. The seawater, which does not appreciably block radio waves in the VLF range, severely attenuates those in the HF band, to act as an effective EMI filter. This submergence method however is of limited effectiveness as it does not work on non-submerged platforms or when the sea state is too high since the VLF loop could then come out of the water due to wave action.
The ultimate effect on submarine operations is that VLF reception has to be postponed until such time as all HF transmission operations have ceased which undesirably extends the time the submarine must remain at periscope depth, thereby increasing its chances of being detected.